It is generally undesirable for steel to have high hydrogen content. Steel having a high hydrogen content is normally characterized by a reduction in its ductility and toughness as compared to low hydrogen content steels. In heavy steel sections, an excessively high hydrogen content may result in the formation of internal cracks during cooling. Such internal cracks are normally referred to in the steelmaking art as flakes, fissures, fisheyes or hairline cracks.
One method known to the steelmaking art for removing hydrogen from a steel melt is vacuum degassing. This method however, requires expensive and hard to maintain equipment, and also requires either a supplementary heat source or a high melt temperature to compensate for heat loss during the vaccumm processing.
The AOD process has achieved wide acceptance in the steel industry due to its ability to decarburize refine the melt, increase productivity, as well as to provide pinpoint temperature and chemistry control for steel melt. While the AOD process will under standard operating practices produce steels having a hydrogen content that is acceptable for most applications, it is not low enough for steel intended to be forged into large sections, such as forgings intended for ship drive shafts and other large section forgings. It is therefore desirable to produce steel for certain applications that has a very low hydrogen content by means of the AOD process.
The AOD process is well known in the art. The basic AOD refining process is disclosed by Krivsky in U.S. Pat. No. 3,252,790. An improvement of Krivsky relating to the programmed blowing of the gases is disclosed by Nelson et al in U.S. Pat. No. 3,046,107. The use of nitrogen in combination with argon and oxygen to achieve predetermined nitrogen contents is disclosed by Saccomano et al in U.S. Pat. No. 3,754,894. An improved AOD Process incorporating a computer program is disclosed in U.S. Pat. No. 3,816,720. A modification of the AOD process is also shown by Johnson et al in U.S. Pat. No. 3,867,135 which utilizes steam or ammonia in combination with oxygen to refine molten metal. U.S. Pat. No. Re. 29,584 discloses an AOD Process wherein the decarburization rate is increased without increasing refractory wear. Choulet and Mehlman disclose in U.S. Pat. No. 4,187,102 a method to control the temperature of a steel melt refined by subsurface pneumatic refining, such as the AOD process. A method for controlling slopping during subsurface pneumatic refining of steel, such as by the AOD process is disclosed by Bury et al in U.S. Pat. No. 4,278,464.
It is therefore an object of this invention to provide an improved AOD Process capable of producing steel having a low hydrogen content.